AARoads Forum

Please login or register.

Login with username, password and session length
Advanced search  

News:

New rules for political content in signatures and user profiles. See this thread for details.

Author Topic: Precast beam bridge observation and question  (Read 888 times)

Tom958

  • *
  • Offline Offline

  • Posts: 976
  • Age: 62
  • Location: Lawrenceville, GA
  • Last Login: July 26, 2021, 06:21:49 AM
Precast beam bridge observation and question
« on: December 24, 2020, 10:32:04 AM »

I've been posting in the ORIGINAL Interstate Bridge Designs From Every State thread, informed by recent research and my own years of roadgeekery. I can be a bit slow on the uptake, but I just realized something...

Most over-the-freeway bridges are either steel plate girders, cast-in-place concrete tee beams (or box beams in some western states), or precast concrete beams. A few years into the construction of the Interstate system, many states increased their typical median width to sixty feet or thereabouts from the former forty to fifty. However, there was a period during which precast beams weren't used on bridges where the medians were that wide. Apparently, there was an upper limit to the viability of precast concrete at the span lengths required for sixty-foot medians. Without specifically re-researching the topic, I suspect that some states delayed the introduction of sixty-foot medians out of their attachment to precast beam bridges. I'm thinking Montana and Iowa off the top of my head, though there are plenty of others I haven't looked at closely.

Having said this, it appears that Florida may be a conspicuous exception to the rule.  This bridge, the first south of the Georgia line on I-75, was built in 1961. The pre-widening median here was 62-64 feet, and the bridge is on a severe skew, as are many others on this stretch.

Obviously, it'd be ridiculous to argue that it was infeasible to build precast beams an extra five feet longer to accommodate an additional ten feet of median width, even if Florida and possibly others hadn't done so early on. Others, of course, followed a few years later. I've seen enough to make me wonder, though.

Oh, I mentioned a question: do you have any knowledge to impart or observations to make about this?
Logged

Dirt Roads

  • *
  • Offline Offline

  • Posts: 908
  • Location: Central North Carolina
  • Last Login: July 27, 2021, 12:02:02 PM
Re: Precast beam bridge observation and question
« Reply #1 on: December 24, 2020, 11:20:47 AM »

I've been posting in the ORIGINAL Interstate Bridge Designs From Every State thread, informed by recent research and my own years of roadgeekery. I can be a bit slow on the uptake, but I just realized something...

Most over-the-freeway bridges are either steel plate girders, cast-in-place concrete tee beams (or box beams in some western states), or precast concrete beams. A few years into the construction of the Interstate system, many states increased their typical median width to sixty feet or thereabouts from the former forty to fifty. However, there was a period during which precast beams weren't used on bridges where the medians were that wide. Apparently, there was an upper limit to the viability of precast concrete at the span lengths required for sixty-foot medians. Without specifically re-researching the topic, I suspect that some states delayed the introduction of sixty-foot medians out of their attachment to precast beam bridges. I'm thinking Montana and Iowa off the top of my head, though there are plenty of others I haven't looked at closely.

Having said this, it appears that Florida may be a conspicuous exception to the rule.  This bridge, the first south of the Georgia line on I-75, was built in 1961. The pre-widening median here was 62-64 feet, and the bridge is on a severe skew, as are many others on this stretch.

Obviously, it'd be ridiculous to argue that it was infeasible to build precast beams an extra five feet longer to accommodate an additional ten feet of median width, even if Florida and possibly others hadn't done so early on. Others, of course, followed a few years later. I've seen enough to make me wonder, though.

Oh, I mentioned a question: do you have any knowledge to impart or observations to make about this?

That puppy's got three sets of support column piers (as compared to one in the middle), so the maximum stringer length on this bridge was still a typical standard design.  But I believe you are correct in your assumption that the usage of AASHTO precast stringers needed to change significantly after the ROW standards changed.  We used longer/thicker stringers sometimes in the AGT (automated guideway transit) industry, but our guideways were usually at a higher elevation than the typical roadway for an Interstate overpass.  We didn't have issues where the additional thickness caused clearance problems, but that is one of the primary factors.

Hopefully, a bridge guru can shed some more light on this issue.
Logged

Tom958

  • *
  • Offline Offline

  • Posts: 976
  • Age: 62
  • Location: Lawrenceville, GA
  • Last Login: July 26, 2021, 06:21:49 AM
Re: Precast beam bridge observation and question
« Reply #2 on: February 08, 2021, 09:16:08 PM »

An unanticipated data point in favor of my hypothesis: In Idaho, the overwhelming majority of Interstates were built with 75-foot medians, and precast beams were the favored material for bridges. Rather than reducing the median or choosing a different material as most other states did, they added an extra span over the median. Here's one example, from 1960, but there are countless others.
Logged

 


Opinions expressed here on belong solely to the poster and do not represent or reflect the opinions or beliefs of AARoads, its creators and/or associates.